The present invention relates to an air/fuel (A/F) ratio sensor for detecting the A/F ratio of an air/fuel mixture being supplied into a combustor. More particularly, the present invention relates to an A/F ratio sensor that is capable of detecting the A/F ratio of an air/fuel mixture using an oxygen ion conductive solid electrolyte over the full operating range, including the lean region (where air is in excess of the stoichiometric value) to the rich region (where fuel is in excess of the stoichiometric value).
With a view to improving fuel economy and reducing emissions, some conventional combustors such as internal combustion engines, have been provided with the capability of feedback control, involving the detection of oxygen levels in the exhaust and control of the air/fuel mixture in the combustion chamber so as to burn it at an A/F ratio in the vicinity of the stoichiometric value. An oxygen sensor commonly used to detect the concentration of oxygen in the exhaust employs an ion conductive solid electrolyte with coatings of porous electrode layers and detects the burning of fuel at an A/F ratio in the vicinity of the stoichiometric value, depending upon the change in the electromotive force generated by the difference between the oxygen partial pressure of the exhaust and that of air. Generally, this type of oxygen sensor produces an output voltage that changes abruptly at the stoichiometric A/F ratio of the air/fuel mixture.
Attempts are being made to maximize the performance of combustors in addition to fuel economy improvements and emissions reduction by means of performing feedback control to attain a desired A/F ratio that is adaptive to a specific state of operation of the combustor. This goal, however, is not attained by the aforementioned oxygen sensor, which is merely capable of detecting the stoichiometric A/F ratio of the air/fuel mixture.
A sensor or analyzer capable of performing the above described A/F ratio feedback control has recently been proposed in Unexamined Published Japanese Patent Application Nos. 72286/1977 and 66292/1978; this device is provided with a chamber that forms a closed space including the surface of one of the two electrodes formed on a solid electrolyte and a small diffusion aperture is formed in the wall of this chamber; a voltage is applied across the two electrodes so that a gas component in the gas to be analyzed will be introduced into the chamber by diffusion; and the amount of current flowing through said solid electrolyte is measured to determine the concentration of the particular gas component.
In the device described above, the atmosphere around one of the two electrodes formed on a solid electrolyte provides a closed space that communicates with the atmosphere of the gas to be analyzed by means of a small diffusion limiting aperture. One major problem with this device is that the diffusion limiting means is difficult to fabricate since the diffusion limited current value must be measured in order to determine the concentration of a particular gas being monitored in the gas to be analyzed.